Where to add client certificates for Docker for Mac?

I have a docker registry that I’m accessing behind an nginx proxy that does authentication using client-side ssl certificates.

When I attempt to push to this registry, I need the docker daemon to send the client certificate to nginx.

According to:
https://docs.docker.com/engine/security/certificates/

There should be a directory called /etc/docker where these certificates can go. This directory doesn’t exist on Docker for Mac.

So I thought I’d try putting the certificates inside the virtual machine itself by doing:

docker-machine ssh default

This resulted in docker complaining:
Error response from daemon: crypto/tls: private key does not match public key

I don’t believe there is anything wrong with my key pair, and I’ve done this same setup on linux (much easier) without problems.